826 



TEXTBOOK OF ZOOLOGY 



yellow-podded pea (dY). The determination of the possible kinds 

 of gametes may be simplified by first making a small checkerboard 

 for them (Fig. 428). Thus we find that in a dihybrid cross there 

 are four kinds of male gametes and four kinds of female gametes 

 which may be listed as TY, Tg, dY, dg. Now we make a checkerboard 

 of sixteen squares and proceed as we did before. (Fig. 429.) 



The computation of the dihybrid cross indicates that nine-sixteenths 

 of the progeny will show the two dominant traits because (Fig. 429) 

 we find at least one T and one Y together in that many squares. Three- 

 sixteenths will sJiow the dominant trait of the first allelomorph and the 

 recessive trait of the other. Another three-sixteenths will show the re- 



Fig. 



Ti 



Tg 



dY 



dg 



429. — The outcome of a dihybrid cross between two heterozygous individuals 

 is according to the ratio 9:3:3:1. 



cessive trait of the first allelomorph and the dominant trait of the sec- 

 ond. One-sixteenth of the offspring will show both recessive traits. It 

 will be noticed, however, that the nine squares showing individuals 

 that will be tall and yellow vary with each other in regard to their 

 entire content. Further examination of the squares will indicate that 

 there are nine different combinations of letters and that in only one 

 case as many as four are exactly similar. Individuals whose genes 

 cause them to look alike are called iihenotypes; those whose genes are 

 exactly alike are called genotypes. 



A trihybrid cross, such as one between a tall yellow-podded pea with 

 a wrinkled seed-coat (TYw) and a dwarf green-podded pea with a 



I 



